1. Field of the Invention
The present invention relates to a printed circuit board of an IC (integrated circuit) package, more particularly, to a printed circuit board on which a semiconductor IC chip (hereinafter referred to simply as a xe2x80x9cchipxe2x80x9d) is flip-chip bonded and which is connected to, i.e., mounted on, a motherboard by means of a plurality of external connection terminals provided on a main surface thereof opposite a chip-mounted main surface.
2. Description of the Related Art
Printed circuit boards of IC packages are widely known to be divided into the ball grid array (BGA) packaging type and the pin grid array (PGA) packaging type. Terminals (hereinafter referred to as xe2x80x9cexternal connection terminals,xe2x80x9d xe2x80x9cconnection terminals,xe2x80x9d or simply xe2x80x9cterminalsxe2x80x9d) provided on a printed circuit board of BGA packaging, and adapted to establish connection to a motherboard, are typically formed in the following manner. Solder balls made of lead-tin solder which has a greater lead content than tin content are fused on solder ball pads provided on a main surface of the printed circuit board by use of solder of a low melting point, such as lead-tin eutectic solder, thereby forming solder bumps as terminals. After a chip is mounted on the printed circuit board, followed by sealing, the printed circuit board is positioned on the motherboard such that connection terminals thereon are aligned with the respective terminals on the motherboard. Then, the facing terminals are fused together through application of heat (hereinafter referred to as xe2x80x9cBGA connectionxe2x80x9d), thereby mounting the printed circuit board on the motherboard.
Meanwhile, terminals on a printed circuit board of PGA packaging are typically formed in the following manner. A number of connection pads are formed through metallization on one main surface of the printed circuit board and are then plated with a predetermined metal. Nail heads of nail-headed pins are brazed to the pads. The pins are plated with nickel or gold to become terminals. The pins are inserted into socket terminals provided on a motherboard, thereby establishing electrical connection between the pins and the respective socket terminals through pressure contact (hereinafter referred to as xe2x80x9cPGA connectionxe2x80x9d).
The above-described printed circuit board of BGA connection allows for provision of a number of external connection terminals at high density, but involves the following problem. Since a printed circuit board of BGA connection is mounted on a motherboard through fusion of solder bumps serving as terminals, the mounted printed circuit board is difficult to demount from the motherboard. Even when the printed circuit board is successfully demounted from the motherboard, the printed circuit board is very difficult, if not impossible, to remount. In other words, the printed circuit board of BGA connection suffers the disadvantage that, once mounted on a motherboard, the printed circuit board is very difficult, if not impossible, to demount from the motherboard.
By contrast, the above-described printed circuit board of PGA connection is mounted on a motherboard through insertion of pins serving as terminals thereof into socket terminals provided on the motherboard. Thus, the mounted printed circuit board can be readily pulled out from the motherboard. However, in order to attach by, for example, brazing, slender pins to pads provided on the printed circuit board, the nail heads of the pins must be of a relatively large size so as to establish the appropriate strength of attachment to the pads. As a result, there are limits on the degree to which the pitch (interval) of pins or terminals can be decreased, in contrast to terminals of the BGA connection type, so that the pins serving as external connection terminals cannot be arrayed as densely as can terminals in the case of BGA connection.
An object of the present invention is to provide a printed circuit board capable of arraying terminals to be connected to those of a motherboard as densely as in the case of a printed circuit board of BGA connection and which can be freely demounted from a motherboard as in the case of a printed circuit board of PGA connection, without deteriorating the reliability of electrical connection.
It is noted the above object is seemingly achievable through employment of conventional solder bumps as terminals to be fitted into socket terminals of a motherboard. However, since the solder bumps are low in corrosion resistance, such connection fails to establish highly reliable electrical connection. Further, the solder bumps cannot be plated with gold (Au) or nickel (Ni) by ordinary plating methods. Also, since the Young""s modulus of lead-tin solder is low, mechanical contact or pressure contact associated with insertion readily causes deformation of the surfaces of the lead-tin solder bumps serving as terminals. Additionally, in the case of repeated mounting and demounting of the printed circuit board to and from a motherboard, the surfaces of the solder bumps are scraped off, resulting in deterioration of electrical connection between the engaged terminals.
To achieve the above object, there is provided, in accordance with the present invention, a printed circuit board comprising a board member and a plurality of ball-shaped terminals provided on a major surface of said board member, the ball-shaped terminals each comprising a pad disposed on the major surface of the board member; a metallic ball (substantially spherical) brazed onto the pad; and a gold plating layer formed on a surface of the metallic ball.
As a result of plating the ball-shaped terminals with gold, high corrosion resistance is imparted to the surfaces of the ball-shaped terminals, so that the fitting of the ball-shaped terminals into the respective socket terminals of a motherboard does not involve impairment of the highly reliable electrical connection provided between the engaged terminals. In other words, the printed circuit board according to the present invention can be freely mounted to or demounted from the motherboard without impairing the reliability of electrical connection between the engaged boards. Also, the ball-shaped terminals can be disposed or arrayed at a satisfactorily high density.
The metallic balls (which hereinafter may be referred to simply as xe2x80x9cballsxe2x80x9d) are advantageously of a metal that permits gold to be deposited, by plating, directly on the metallic-ball surface or deposited indirectly, i.e., deposited on a nickel plating layer formed on the metallic-ball surface. Preferably, the metallic balls are formed of a material selected so as to render the ball-shaped terminals resistant to deformation which would otherwise result from the insertion thereof into the socket terminals of a motherboard, i.e., so as to render the ball-shaped terminals free from any plastic deformation which would otherwise result from their insertion into the socket terminals and would thus cause or result in poor conduction of electricity. Metallic balls of a soft metal, such as lead-tin solder, deform easily when subjected to repeated mounting and demounting, resulting in deterioration in the reliability of electrical connection. Through employment of such a deformation-resistant metal for the metallic balls, the printed circuit board becomes highly reliable.
The material for the metallic balls is preferably selected according to a motherboard to be used; specifically, based on the compressive force which the socket terminals of the motherboard exert on the metallic balls fitted therein. Preferably, from the viewpoint of cost and electrical conductivity, the metallic balls are made of copper (Cu) or a copper alloy. From the viewpoint of Young""s modulus, an iron-nickel alloy, such as Kovar or 42-alloy, is preferred. However, the material for the metallic balls is not limited to these examples, but may be selected as appropriate.
Preferably, from the viewpoint of adhesion, the gold plating layer is formed on an Ni plating layer formed on the metallic-ball surface. The thickness of the gold plating layer may be selected as appropriate, but is preferably 0.03 xcexcm to 3 xcexcm from the viewpoint of adhesion and economy. More preferably, from the viewpoint of factors related to reliability of electrical connection, such as resistance to corrosion or abrasion, the thickness of the gold plating layer is 1 xcexcm to 3 xcexcm. A brazing material for brazing the metallic balls on the respective pads is preferably a silver brazing filler metal, which is compatible with plating with gold.
In order to securely engage the ball-shaped terminals and the socket terminals, the ball-shaped terminals are preferably shaped such that, once engaged with the respective socket terminals, the metallic balls function as clamps against disengagement. To this end, the metallic balls preferably have a diameter greater than that of the pads and are brazed on the respective pads substantially coaxially therewith. As a result, when the metallic ball is brazed on the pad, a constricted portion (a smaller-diameter portion) is formed at a brazed portion of the resultant ball-shaped terminal. Accordingly, when the socket terminals of a motherboard are shaped so as to hold the respective ball-shaped terminals at the constricted portions, the ball-shaped terminals engaged with the respective socket terminals are secured in place.